Detects invalid encodings of bls12381 elements.

cloudflare · Jun 10, 2024 · 5c44dde · 5c44dde
1 parent d556030
commit 5c44dde
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Showing 3 changed files with 49 additions and 19 deletions.
diff --git a/ecc/bls12381/doc.go b/ecc/bls12381/doc.go
@@ -32,23 +32,41 @@
 // is the lexicographically largest of the two associated with the encoded
 // x-coordinate.
 //
-//	|----------------------------------------------------|
-//	|                Serialization Format                |
-//	|-----|-------|-------|---------------|--------------|
-//	| MSB | MSB-1 | MSB-2 |  Description  | Encoding     |
-//	|-----|-------|-------|---------------|--------------|
-//	|  0  |   X   |   X   | Uncompressed  |  e || x || y |
-//	|  1  |   X   |   X   | Compressed    |  e || x      |
-//	|-----|-------|-------|---------------|--------------|
-//	|  X  |   0   |   X   | Non-Infinity  |  e || x || y |
-//	|  X  |   1   |   X   | Infinity      |  e || 0 || 0 |
-//	|-----|-------|-------|---------------|--------------|
-//	|     |       |       | Compressed,   |              |
-//	|  1  |   0   |   1   | Non-Infinity, |  e || x      |
-//	|     |       |       | Big y-coord   |              |
-//	|-----|-------|-------|---------------|--------------|
-//	|     |       |       | Compressed,   |              |
-//	|  1  |   0   |   0   | Non-Infinity, |  e || x      |
-//	|     |       |       | Small y-coord |              |
-//	|----------------------------------------------------|
+//	|------------------------------------------------------|
+//	|                 Serialization Format                 |
+//	|-----|-------|-------|-----------------|--------------|
+//	| MSB | MSB-1 | MSB-2 |   Description   |   Encoding   |
+//	|-----|-------|-------|-----------------|--------------|
+//	|     |       |       | Non-compressed, |              |
+//	|  0  |   0   |   0   | Non-Infinity,   |  e || x || y |
+//	|     |       |       | Zero.           |              |
+//	|-----|-------|-------|-----------------|--------------|
+//	|     |       |       | Non-compressed, |              |
+//	|  0  |   0   |   1   | Non-Infinity,   |   Invalid    |
+//	|     |       |       | One.            |              |
+//	|-----|-------|-------|-----------------|--------------|
+//	|     |       |       | Non-compressed, |              |
+//	|  0  |   1   |   0   | Infinity,       |  e || 0 || 0 |
+//	|     |       |       | Zero.           |              |
+//	|-----|-------|-------|-----------------|--------------|
+//	|     |       |       | Non-compressed, |              |
+//	|  0  |   1   |   1   | Infinity,       |   Invalid    |
+//	|     |       |       | One.            |              |
+//	|-----|-------|-------|-----------------|--------------|
+//	|     |       |       | Compressed,     |              |
+//	|  1  |   0   |   0   | Non-Infinity,   |  e || x      |
+//	|     |       |       | Small y-coord   |              |
+//	|-----|-------|-------|-----------------|--------------|
+//	|     |       |       | Compressed,     |              |
+//	|  1  |   0   |   1   | Non-Infinity,   |  e || x      |
+//	|     |       |       | Big y-coord     |              |
+//	|-----|-------|-------|-----------------|--------------|
+//	|     |       |       | Compressed,     |              |
+//	|  1  |   1   |   0   | Infinity,       |  e || 0      |
+//	|     |       |       | Zero.           |              |
+//	|-----|-------|-------|-----------------|--------------|
+//	|     |       |       | Compressed,     |              |
+//	|  1  |   1   |   1   | Infinity,       |   Invalid    |
+//	|     |       |       | One.            |              |
+//	|------------------------------------------------------|
 package bls12381
diff --git a/ecc/bls12381/g1.go b/ecc/bls12381/g1.go
@@ -34,6 +34,12 @@ func (g *G1) SetBytes(b []byte) error {
 		return errInputLength
 	}
 
+	// Check for invalid prefixes
+	switch b[0] & 0xE0 {
+	case 0x20, 0x60, 0xE0:
+		return errEncoding
+	}
+
 	isCompressed := int((b[0] >> 7) & 0x1)
 	isInfinity := int((b[0] >> 6) & 0x1)
 	isBigYCoord := int((b[0] >> 5) & 0x1)

diff --git a/ecc/bls12381/g2.go b/ecc/bls12381/g2.go
@@ -32,6 +32,12 @@ func (g *G2) SetBytes(b []byte) error {
 		return errInputLength
 	}
 
+	// Check for invalid prefixes
+	switch b[0] & 0xE0 {
+	case 0x20, 0x60, 0xE0:
+		return errEncoding
+	}
+
 	isCompressed := int((b[0] >> 7) & 0x1)
 	isInfinity := int((b[0] >> 6) & 0x1)
 	isBigYCoord := int((b[0] >> 5) & 0x1)